This PR is a prerequisite for #31342

It adds a new option to filter notifications from limited (aka “silenced”) accounts, and extends each option with the ability to drop notifications rather than filter them.

This deprecates /api/v1/notifications/policy (which was never released) and replaces it with:

GET /api/v2/notifications/policy

{
  "for_not_following": "accept",
  "for_not_followers": "accept",
  "for_new_accounts": "accept",
  "for_private_mentions": "filter",
  "for_limited_accounts": "filter",
  "summary": {
    "pending_requests_count": 1,
    "pending_notifications_count": 1
  }
}

TODO

• add model and database support
• adapt old API endpoint
• apply new options in NotifyService
• add data migration (pre-migration and post-migration)
• add new API endpoint